The flyback topology is, arguably, one of the most used circuit topologies in the field of power conversion, especially in lower- to medium-power applications (such as AC-DC adapters, for example). The reason for high level of utilization of the flyback topology is rooted in its simplicity and low cost of implementation, as well as in the fact that the so-configured electrical circuitry can operate efficiently over a very large range of input voltage. In AC-DC adapter applications with powers under about 70 W, in order to gain a capability to be applicable substantially universally all over the worlds, the circuits formatted according to the flyback topology are used to operate after an output from a simple bridge rectifier, while the alternating-current input voltage ranges from 90 Vac to 264 Vac. (Conventionally, a rectifier is known an electrical device that converts alternating current to direct current, which flows in only one direction.)
To meet all the AC-voltage standards for different countries, when placed at the output of the rectifier, in the flyback converter has to be able to operate efficiently with a DC input voltage ranging from 127 Vdc to 375 Vdc (which is a range in which the ratio of the upper input voltage limit to the lower voltage limit is almost 3:1.) In addition to that, the new standards for power delivery require that the adapters provide a voltage output ranging from 5V to 20V (with the ration of the upper voltage limit to the lower voltage limit of 4:1, as far as the output voltage is concerned). Most of the forward-derived topologies (such as, for example, half-bridge topology, two-transistor forward topology, full bridge topology, to name just a few) are not able to operate efficiently over such large input and output voltage ranges provided by the transfer function of the flyback topology based circuit.
The trend for miniaturization of portable equipment (for example, portable computing devices such as laptops and tablets, for example) extends this demand even further, as a result of which the AC-DC adapters also became subject to these requirements. Presently, most of the laptops and tablets require, for operation, power ranging from 30 W to 65 W. The significant technological advancement in portable computing devices, the size of laptops and tablets has been significantly reduced while the AC-DC adapters used to power such devices remain quite large (for example, dimensions of a typical adaptor for a small tablet device are about 3.3″ by 1.8″ by 1.3″ or so). This has created pressures for the size reduction of the AC-DC adapters. An ability to reduce the size of the required adapters while maintaining the convection-based cooling methodology used today requires some significant improvement in efficiency of the adapters as well as decrease of size of the magnetic and capacitive storage elements.
Over the years, the efficiency of the AC-DC adapters has been increased from about 70% to about 89-90% (in the most recent products such as the Apple 30 W adapter, for example), mostly due to the significant progress in semiconductor industry and a better understanding of magnetic technology. The flyback topology, however, possesses several drawbacks that limit its efficiency of operation. In most of the application the flyback-topology circuitry operates in a discontinuous mode. In a discontinuous mode of operation, the magnetizing current is first built up from zero to a peak level during the time period when the main switch is conducting; and after the main switch turns off, the magnetizing current flows into the secondary side winding and transfers the energy to the output capacitor until the value of the magnetizing current decreases to zero. This portion of the operation cycle is followed by a second period of time, referred as “dead time”, when no energy is stored in the transformer or transferred to the secondary. Together, the first and second period of time characterize the discontinuous mode of operation of the flyback-topology circuits. When the “dead time” is reduced to the transition time which is the time interval wherein the voltage across the main switch decays from the level it had during the time when the magnetizing current flows into the secondary winding to its lowest level which occurs in the beginning of “dead time”, this mode of operation is referred as a critical conduction mode of operation.
This disclosure presents several electronic-circuitry configurations that address the limitations conventionally associated with the flyback topology. The proposed solutions increase the efficiency of the flyback-topology-utilizing power converters above about 94%, decrease the level of dissipated heat and, as a result, produce a much higher power density (for example, above 27 W/in3).